Discharge of a thermal storage tank using an immersed heat exchanger with an annular baffle

A number of solar domestic hot water systems and many combined space and water heating systems have heat exchangers placed directly in the storage fluid to charge and/or discharge the tank. Operation of the heat exchanger produces a buoyancy-driven flow within the storage fluid. With a view toward controlling the flow field to increase heat transfer, a cylindrical baffle is inserted in a 350 l cylindrical storage tank. The baffle creates a 40 mm annular gap adjacent to the tank wall. A 10 m-long, 0.3 m² copper coil heat exchanger is placed in the gap. The effects of the baffle on the transient heat transfer, delivered water temperature, heat exchanger effectiveness, and temperature distribution within the storage fluid are presented during discharge of initially thermally stratified and fully mixed storage tanks. The baffle increases the storage side convective heat transfer to the heat exchanger by 20%. This increase is attributed to higher storage fluid velocities across the heat exchanger.     

Design and experimental testing of the performance of an outdoor LiBr/H2O solar thermal absorption cooling system with a cold store

A domestic-scale prototype experimental solar cooling system has been developed based on a LiBr/H₂O absorption system and tested during the 2007 summer and autumn months in Cardiff University, UK. The system consisted of a 12 m² vacuum tube solar collector, a 4.5 kW LiBr/H₂O absorption chiller, a 1000 l cold storage tank and a 6 kW fan coil. The system performance, as well as the performances of the individual components in the system, were evaluated based on the physical measurements of the daily solar radiation, ambient temperature, inlet and outlet fluid temperatures, mass flow rates and electrical consumption by component. The average coefficient of thermal performance (COP) of the system was 0.58, based on the thermal cooling power output per unit of available thermal solar energy from the 12 m² Thermomax DF100 vacuum tube collector on a hot sunny day with average peak insolation of 800 W/m² (between 11 and 13.30 h) and ambient temperature of 24 °C. The system produced an electrical COP of 3.6. Experimental results prove the feasibility of the new concept of cold store at this scale, with chilled water temperatures as low as 7.4 °C, demonstrating its potential use in cooling domestic scale buildings.     

Performance of a non-metallic unglazed solar water heater with integrated storage system

The performance of a new design of non-metallic unglazed solar water heater integrated with a storage system has been studied. In this system, the collector and storage were installed in one unit. All parts of the system have been fabricated from fiberglass reinforced polyester (GFRP) using a special resin composition that provides good thermal conductivity and absorptivity. The storage tank has a capacity of 329 l. The design of the storage system was sandwich construction, with the core material made out of polyurethane foam, which combines stiffness and lightness of structure with very good thermal insulation. The width and length of the absorber plat were 1.4 and 1.8 m, respectively. The performance of the system has been investigated by two methods. In the first method, the storage tank was filled up with water the night before the test. The tank was then drained during the night, refilled and made ready for the next day’s test. The tests were repeated under varied environmental conditions for several days. The maximum water temperature in the storage tank of 63 °C has been achieved for a clear day operation at an average solar radiation level of 700 W m⁻² and ambient temperature of 30 °C. The decrease of water temperature with and without the thermal diode is 10 and 20 °C, respectively. In the second method, the testing was of the same way, but in this case without draw-off or draining of the hot water from the storage tank. All data readings were recorded from sunrise to sunset over the same period. The temperature was recorded for several days and ranges of 60–63 °C were obtained in the storage tank. A system efficiency of 45% was achieved at an average solar radiation level of 635 W m⁻² and ambient temperature of 31 °C.     

Concentrated solar power on demand

A concentrating solar power system is presented which uses hillside mounted heliostats to direct sunlight into a volumetric absorption molten salt receiver with integral storage. The concentrated sunlight penetrates and is absorbed by molten salt in the receiver through a depth of 4-5 m, making the system insensitive to the passage of clouds. The receiver volume also acts as the thermal storage volume eliminating the need for secondary hot and cold salt storage tanks. A small aperture and refractory-lined domed roof reduce losses to the environment and reflect thermal radiation back into the pond. Hot salt is pumped from the top of the tank through a steam generator and then returned to the bottom of the tank. An insulated barrier plate is positioned within the tank to provide a physical and thermal barrier between the thermally stratified layers, maintaining hot and cold salt volumes required for continuous operation. As a result, high temperature thermal energy can be provided 24/7 or at any desired time.The amount of storage required depends on local needs and economic conditions. About 2500 m³ of nitrate salt is needed to operate a 4 MW_e steam turbine 24/7 (7 h sunshine, 17 h storage), and with modest heliostat field oversizing to accumulate energy, the system could operate for an additional 24 h (1 cloudy day). Alternatively, this same storage volume can supply a 50 MW_e turbine for 3.25 h without additional solar input. Cosine effect losses associated with hillside heliostats beaming light downwards to the receiver are offset by the elimination of a tower and separate hot and cold storage tanks and their associated pumping systems. Reduced system complexity also reduces variable costs. Using the NREL Solar Advisor program, the system is estimated to realize cost-competitive levelized production costs of electricity.     

A new version of a solar water heating system coupled with a solar water pump

This research target was to improve the thermal efficiency of a solar water heating system (SWHS) coupled with a built-in solar water pump. The designed system consists of 1.58-m² flat plate solar collectors, an overhead tank placed at the top level, the larger water storage tank without a heat exchanger at the lower level, and a one-way valve for water circulation control. The discharge heads of 1 and 2 m were tested. The pump could operate at the collector temperature of about 70–90 °C and vapor gage pressure of 10–18 kPa. It was found that water circulation within the SWHS ranged between 15 and 65 l/d depending upon solar intensity and discharge head. Moreover, the max water temperature in the storage tank is around 59 °C. The max daily pump efficiency is about 0.0017%. The SWHS could have max daily thermal efficiency of about 21%. It is concluded that the thermal efficiency was successfully improved, except for the pump one. The new SWHS with 1 m discharge head or lower is suitable for residential use. It adds less weight to a building roof and saves electrical energy for a circulation pump. It has lower cost compared to a domestic SWHS.     

Performance calculations for closed-loop air-to-water solar hybrid heating systems with and without a rock bed in the solar air heater

A transient analysis has been carried out on a hybrid solar water heater which comprises a rock bed air heater with optimum design parameters in conjunction with an air-to-water transverse fin shell-and-tube heat exchanger (mixed air and unmixed water type) in which cold water from the storage tank receives heat from the hot air coming out of the air heater which flows in the shell at right angles to the water flowing in the tubes. The system's performance has been evaluated for typical winter weather conditions in Delhi for different combinations of flow rates of air and water for different volumes of the water storage tank. No hot water is assumed to be withdrawn from the tank to serve the load. A comparative analysis of the system's performance with and without a rock bed in the air heater reveals about 11°C higher temperature of storage tank in the former at 50 kg/h m² air flow rate. With both the air heater types, although the system performance was observed to increase with the rates of air and water flow, no significant improvement in system performance was achieved at .     

Effects of auxiliary heater on annual performance of thermosyphon solar water heater simulated under variable operating conditions

A thermosyphon solar water heating system with electric auxiliary heater was simulated using the TRNSYS simulation program. Location of the auxiliary heater, inside the storage tank or connected in series between the system and the user, was studied using the TMY meteorological data for Los Angeles, California. Simulations were performed for two different water load temperatures (60 and 80°C) and for two types of daily hot water volumes (250 and 150 l). Four types of daily hot water consumption profiles were used in the present study, namely; the widely used Rand profile, continuous, evening and morning profiles. Also, the simulation is extended to cover the effects of thermal and optical properties of the flatplate collector and the volume of the storage tank. The results show that if water is drawn on a schedule corresponding to the Rand draw profile, the system operates with higher efficiency when the auxiliary heater is located in the storage tank than when the auxiliary heater is outside the storage tank. When operated with each of the other three draw schedules, however, better performance is achieved by locating the auxiliary heater outside the tank. The increase in solar fraction depends on the load profile and volume, temperature setting, as well as the quality of the collector and the storage tank volume. When the values of the parameters F_R(τα)_n and F_RU_L are changed from 0.8 and 16 kJ/h m²°C to 0.6 and 30 kJ/h m²°C, the solar fraction decreases by approximately 40–50%.     

Experimental investigation and theoretical analysis of the solar adsorption cooling system in a green building

A solar adsorption cooling system was constructed in the green building of Shanghai Institute of Building Science. The system consisted of evacuated tube solar collector arrays of area 150 m², two adsorption chillers with nominal cooling capacity of 8.5 kW for each and a hot water storage tank of 2.5 m³ in volume. A mathematical model of the system was established. According to experimental results under typical weather condition of Shanghai, the average cooling capacity of the system was 15.3 kW during continuous operation for 8 h. The theoretical analysis of the system was verified and found to agree well with the experimental results. The performance analysis showed that solar radiant intensity had a more distinct influence on the performance of solar adsorption cooling system as compared with ambient temperature. It was observed that the cooling capacity increased with the increase of solar collector area, whereas, solar collecting efficiency varied quite contrary. With the increase of water tank volume, cooling capacity decreased, while, the solar collecting efficiency increased. The system performances can be enhanced by increasing the height-to-diameter ratio of water tank. Additionally, it was observed that solar collecting efficiency decreased with the increase of the initial temperature of water in the tank; however, cooling capacity varied on the contrary. Also can be seen is that optimum nondimensional mass flow rate is 0.7 when the specific mass flow rate exceeds 0.012 kg/m² s.     

Performance of a multi-functional direct-expansion solar assisted heat pump system

This paper reports on the long-term performance of a direct-expansion solar assisted heat pump (DX-SAHP) system for domestic use, which can offer space heating in winter, air conditioning in summer and hot water during the whole year. The system employs a bare flat-plate collector array with a surface area of 10.5 m², a variable speed compressor, a storage tank with a total volume of 1 m³ and radiant floor heating unit. The performance under different operation modes is presented and analyzed in detail. For space-heating-only mode, the daily-averaged heat pump COP varied from 2.6 to 3.3, while the system COP ranged from 2.1 to 2.7. For water-heating-only mode, the DX-SAHP system could supply 200 l or 1000 l hot water daily, with the final temperature of about 50 °C, under various weather conditions in Shanghai, China. For space-cooling-only mode, the compressor operates only at night to take advantage of a utility’s off-peak electrical rates by chilling water in the thermal storage tank for the daytime air-conditioning. It shows that, the multi-functional DX-SAHP system could guarantee a long-term operation under very different weather conditions and relatively low running cost for a whole year.     

Thermal analysis and performance optimization of a solar hot water plant with economic evaluation

The main objective of this study is to optimize the long-term performance of an existing active-indirect solar hot water plant (SHWP), which supplies hot water at 65 °C for use in a flight kitchen, using a micro genetic algorithm in conjunction with a relatively detailed model of each component in the plant and solar radiation model based on the measured data. The performance of SHWP at Changi International Airport Services (CIASs), Singapore, is studied for better payback period using the monthly average hourly diffuse and beam radiations and ambient temperature data. The data input for solar radiation model is obtained from the Singapore Meteorological Service (SMS), and these data have been compared with long-term average data of NASA (surface meteorology and solar energy or SSE). The comparison shows a good agreement between the predicted and measured hourly-averaged, horizontal global radiation.The SHWP at CIAS, which comprises 1200 m² of evacuated-tube collectors, 50 m³ water storage tanks and a gas-fired auxiliary boiler, is first analyzed using a baseline configuration, i.e., (i) the local solar insolation input, (ii) a coolant flow rate through the headers of collector based on ASHRAE standards, (iii) a thermal load demand pattern amounting to 100 m³/day, and (iv) the augmentation of water temperature by auxiliary when the supply temperature from solar tank drops below the set point. A comparison between the baseline configuration and the measured performance of CIAS plant gives reasonably good validation of the simulation code. Optimization is further carried out for the following parameters, namely; (i) total collector area of the plant, (ii) storage volume, and (iii) three daily thermal demands. These studies are performed for both the CIAS plant and a slightly modified plant where the hot water supply to the load is adjusted constant at times when the water temperature from tank may exceed the set temperature. It is found that the latter configuration has better thermal and economic performances over the conventional design.     

Collector cum storage solar water heaters with and without transparent insulation material

The integrated collector–storage solar water heaters are less expensive and can offer the best alternative for domestic applications particularly to small families to meet hot water requirements. The top heat losses of such solar water heaters are quite high during the night and the temperature of stored hot water is considerably reduced unless covered with extra insulating cover in the evening which is a cumbersome job. The transparent insulation material widely used in Europe for space heating can also minimize top heat losses, if used in such solar water heaters. For this purpose, two units of solar collector cum storage water heaters have been designed to study the relative effect of TI for retaining solar heated hot water for a night duration. Both units were identical in all respects except one of them was covered with TIM. The theoretical exercise was carried out to evaluate design parameters of ISC which revealed total heat loss factor (U_L) 1.03 W/m² K with TIM glazed against 7.06 W/m² K with glass glazed. The TIM glazed has been found to be quite effective as compared to glass glazed SWH and yielded hot water at higher temperature by 8.5 to 9.5°C the next morning. The storage efficiency of such solar water heaters has been found to be 39.8% with TIM glazed as compared to 15.1% without TIM. The TIM glazing means not having to cover the ISC solar water heater with a separate insulator cover in the evening and thus makes its operation much simpler.     

An investigation of the solar powered absorption refrigeration system with advanced energy storage technology

This paper presented a new solar powered absorption refrigeration (SPAR) system with advanced energy storage technology. The advanced energy storage technology referred to the Variable Mass Energy Transformation and Storage (VMETS) technology. The VMETS technology helped to balance the inconsistency between the solar radiation and the air conditioning (AC) load. The aqueous lithium bromide (H₂O-LiBr) was used as the working fluid in the system. The energy collected from the solar radiation was first transformed into the chemical potential of the working fluid and stored in the system. Then the chemical potential was transformed into thermal energy by absorption refrigeration when AC was demanded. In the paper, the working principle and the flow of the SPAR system were explained and the dynamic models for numerical simulation were developed. The numerical simulation results can be used to investigate the behavior of the system, including the temperature and concentration of the working fluid, the mass and energy in the storage tanks, the heat loads of heat exchanger devices and so on. An example was given in the paper. In the example, the system was used in a subtropical city like Shanghai in China and its operating conditions were set as a typical summer day: the outdoor temperature varied between 29.5 °C and 38 °C, the maximum AC load was 15.1 kW and the total AC capacity was 166.1 kW h (598.0 MJ). The simulation results indicated that the coefficient of performance (COP) of the system was 0.7525 or 0.7555 when the condenser was cooled by cooling air or by cooling water respectively and the storage density (SD) was about 368.5 MJ/m³. As a result, the required solar collection area was 66 m² (cooling air) or 62 m² (cooling water) respectively. The study paves the road for system design and operation control in the future.     

Neural network modelling of thermal stratification in a solar DHW storage

In this study an artificial neural network (ANN) model is introduced for modelling the layer temperatures in a storage tank of a solar thermal system. The model is based on the measured data of a domestic hot water system. The temperatures distribution in the storage tank divided in 8 equal parts in vertical direction were calculated every 5 min using the average 5 min data of solar radiation, ambient temperature, mass flow rate of collector loop, load and the temperature of the layers in previous time steps. The introduced ANN model consists of two parts describing the load periods and the periods between the loads. The identified model gives acceptable results inside the training interval as the average deviation was 0.22 °C during the training and 0.24 °C during the validation.     

A review of: “World Energy Supply: Resource,Technologies” Perspectives MANFRED GRATHWOHL

A theoretical model for unsteady one-dimensional temperature field inside the hot-water storage tank of a thermosyphonic solar system is carried out. Hot water flows into a vertical storage tank via its top central line at low constant rates. The measured temperature profiles along the central axis inside the storage tank of a V-trough thermosyphonic solar heater are observed to be in line with the analytical predictions. The linearity of the temperature profiles as checked experimentally is distinctly observed for high inflow velocities of the hot water into the storage tank.     

Experimental studies of a new solar water heater system using a solar water pump

The research goal was to develop a new solar water heater system (SWHS) that used a solar water pump instead of an electric pump. The pump was powered by the steam produced from a flat plate collector. Therefore, heat could be transferred downward from the collector to a hot water storage tank. The designed system consisted of four panels of flat plate solar collectors, an overhead tank installed at an upper level and a large water storage tank with a heat exchanger at a lower level. Discharge heads of 1, 1.5 and 2 m were tested. The pump could operate at the collector temperature of about 70–90 °C and vapor gage pressure of 7–14 kPa. It was found that water circulation within the SWHS ranged between 12 and 59 l/d depending on the incident solar intensity and system discharge head. The average daily pump efficiency was about 0.0014–0.0019%. Moreover, the SWHS could have a daily thermal efficiency of about 7–13%, whereas a conventional system had 30–60% efficiency. The present system was economically comparable to a conventional one.     

A truncated pyramid non-tracking type multipurpose domestic solar cooker/hot water system

For impressive dissemination of the solar thermal gazettes, it is imperative to keep on changing the device design features so as to cater to the different demands of diverse section of the society. Domestic solar hot water systems are not suitable for cooking and the capacity of domestic solar box type cookers for water heating is very low. We report truncated pyramid geometry based multipurpose solar device which could be used for domestic cooking as well as water heating. The device is designed, fabricated and tested. Cooking tests approved by Bureau of Indian Standards were performed in different seasons and the device was found to meet the requirement stipulated on two figures of merit. The performance of the design was also evaluated as a hot water system and the maximum efficiency was found to be 54%. The day-time and average night-time heat-loss coefficients were found to be 5.7 W/°C m² and, 3.74 W/C m², respectively, which are comparable to those of flat-plate collector based solar hot water systems. A simple economic analysis illustrate that this kind of multi-purpose design could be financially viable and physically useful.     

Experimental investigation and simulation analysis of the thermal performance of a balcony wall integrated solar water heating unit

A balcony wall type solar water heater system was designed and constructed in a high-rise building. The U-type evacuated glass tube solar collector is fixed vertically on the balcony wall. The water, heated in the solar collector, flows through the exchanger coil in the water tank and then flows back to the solar collector. With regard to the hot water supply system, the cold water, heated by the heat exchanger, is sent to the point of use. Considering storeys and water consumption pattern, four apartments are selected for testing. Meanwhile, the theoretical analysis with TRNSYS was presented. According to the experimental results, mean daily collector efficiency is about 40%. Solar fraction is high in summer and autumn for the relative high radiation and high ambient temperature. Under given conditions, the annual energy extracted from tank is 2805.3 MJ/m², and the annual solar fraction is 40.5%. When the tank volume-to-collector area ratio is decreased to 37.5 L/m², the solar fraction can be increased to 50%. The results show that the family to use water all day round gets higher solar fraction than the family using hot water mostly in the morning and night.     

Photovoltaic-thermal collectors for night radiative cooling of buildings

A new photovoltaic-thermal (PVT) system has been developed to produce electricity and cooling energy. Experimental studies of uncovered PVT collectors were carried out in Stuttgart to validate a simulation model, which calculates the night radiative heat exchange with the sky. Larger PVT frameless modules with 2.8 m² surface area were then implemented in a residential zero energy building and tested under climatic conditions of Madrid. Measured cooling power levels were between 60 and 65 W m⁻², when the PVT collector was used to cool a warm storage tank and 40-45 W m⁻², when the energy was directly used to cool a ceiling. The ratio of cooling energy to electrical energy required for pumping water through the PVT collector at night was excellent with values between 17 and 30. The simulated summer cooling energy production per square meter of PVT collector in the Madrid/Spain climatic conditions is 51 kWh m⁻² a⁻¹. In addition to the thermal cooling gain, 205 kWh m⁻² a⁻¹ of AC electricity is produced under Spanish conditions. A comparative analysis for the hot humid climate of Shanghai gave comparable results with 55 kWh m⁻² a⁻¹ total cooling energy production, mainly usable for heat rejection of a compression chiller and a lower electricity production of 142 kWh m⁻² a⁻¹.     

Performance investigation of a salt gradient solar pond coupled with desalination facility near the Dead Sea

Solar ponds provide the most convenient and least expensive option for heat storage for daily and seasonal cycles. This is particularly important for a desalination facility, if steady and constant water production is required. If, in addition to high storage capacity, other favorable conditions exist, the salt gradient solar ponds (SGSPs) are expected to be able to carry the entire load of a large-scale flash desalination plants without dependence upon supplementary sources. This paper presents a performance investigation of a SGSP coupled with desalination plant under Jordanian climatic conditions. This is particularly convenient in the Dead Sea region characterized by high solar radiation intensities, high ambient temperature most of the year, and by the availability of high concentration brine. It was found that a 3000 m² solar pond installed near the Dead Sea is able to provide an annual average production rate of 4.3 L min⁻¹ distilled water compared with 3.3 L min⁻¹ that would be produced by El Paso solar pond, which has the same surface area. Based on this study, solar ponds appear to be a feasible and an appropriate technology for water desalination near the Dead Sea in Jordan.     

A solar thermal cooling and heating system for a building: Experimental and model based performance analysis and design

A solar thermal cooling and heating system at Carnegie Mellon University was studied through its design, installation, modeling, and evaluation to deal with the question of how solar energy might most effectively be used in supplying energy for the operation of a building. This solar cooling and heating system incorporates 52 m² of linear parabolic trough solar collectors; a 16 kW double effect, water-lithium bromide (LiBr) absorption chiller, and a heat recovery heat exchanger with their circulation pumps and control valves. It generates chilled and heated water, dependent on the season, for space cooling and heating. This system is the smallest high temperature solar cooling system in the world. Till now, only this system of the kind has been successfully operated for more than one year. Performance of the system has been tested and the measured data were used to verify system performance models developed in the TRaNsient SYstem Simulation program (TRNSYS). On the basis of the installed solar system, base case performance models were programmed; and then they were modified and extended to investigate measures for improving system performance. The measures included changes in the area and orientation of the solar collectors, the inclusion of thermal storage in the system, changes in the pipe diameter and length, and various system operational control strategies. It was found that this solar thermal system could potentially supply 39% of cooling and 20% of heating energy for this building space in Pittsburgh, PA, if it included a properly sized storage tank and short, low diameter connecting pipes. Guidelines for the design and operation of an efficient and effective solar cooling and heating system for a given building space have been provided.